Re: joining two tables slow due to sequential scan

ok here is real db

the first query I had seems to make no sense because it is only fast if
I limit the rows since almost all rows have status = 'AC'

second query
tables both have about 10 million rows and it takes a long time as you
can see but this person only has approx 160 total documents

QUERY PLAN
------------------------------------------------------------------------
-------------------------------------------------------------------
Hash Join (cost=84813.14..1510711.97 rows=48387 width=555) (actual
time=83266.854..91166.315 rows=3 loops=1)
Hash Cond: ("outer".documentidentifier =
"inner".dssdocumentidentifier)
-> Seq Scan on documentversions (cost=0.00..269141.98 rows=9677398
width=415) (actual time=0.056..49812.459 rows=9677398 loops=1)
-> Hash (cost=83660.05..83660.05 rows=48036 width=140) (actual
time=10.833..10.833 rows=3 loops=1)
-> Bitmap Heap Scan on clinicaldocuments
(cost=301.13..83660.05 rows=48036 width=140) (actual time=0.243..0.258
rows=3 loops=1)
Recheck Cond: (patientidentifier = 690193)
-> Bitmap Index Scan on ix_cdocpid (cost=0.00..301.13
rows=48036 width=0) (actual time=0.201..0.201 rows=3 loops=1)
Index Cond: (patientidentifier = 690193)
Total runtime: 91166.540 ms

Tim Jones
Healthcare Project Manager
Optio Software, Inc.
(770) 576-3555

-----Original Message-----
From: Tom Lane [mailto:tgl(at)sss(dot)pgh(dot)pa(dot)us]
Sent: Friday, February 10, 2006 5:52 PM
To: Tim Jones
Cc: Scott Marlowe; Dave Dutcher; pgsql-performance(at)postgresql(dot)org
Subject: Re: [PERFORM] joining two tables slow due to sequential scan

"Tim Jones" <TJones(at)optio(dot)com> writes:
> QUERY PLAN
> 'Hash Join (cost=899.83..4384.17 rows=482 width=1350) (actual
> time=0.203..0.203 rows=0 loops=1)'
> ...
> 'Total runtime: 0.392 ms'

Hardly seems like evidence of a performance problem ...

regards, tom lane